The invention relates in general to antenna elements. More specifically, the invention relates to an antenna structure that exhibits circular polarization for wireless communications devices.
A variety of prior art antennas are currently used in wireless communication devices. One type of antenna is an external half wave single or multi-band dipole. This antenna typically extends or is extensible from the body of a wireless communication device (WCD) in a linear fashion. While this type of antenna is acceptable for use in conjunction with some WCDs, several drawbacks impede greater acceptance and use of such external half wave single or multiband dipole antennas. One significant drawback is that the antenna is typically mounted at least partially external to the body of a WCD which places the antenna in an exposed position where it may be accidentally or deliberately damaged, bent, broken, or contaminated.
Furthermore, due to the physical configuration of this class of onni-directional antenna, optimizing performance for a particular polarization and/or directional signal is not an option. That is, these types of prior art antennas are relatively insensitive to directional signal optimization or, said another way, these types of prior art antennas can operate in a variety of positions relative to a source signal without substantial signal degradation. This performance characteristic is often known as an xe2x80x9comni-directionalxe2x80x9d quality, or characteristic, of signal receipt and transmission. This means that electromagnetic waves radiate substantially equally in all directions during transmitting operations. Such prior art antennas also are substantially equally sensitive to receiving signals from any given direction (assuming adequate signal strength). Unfortunately, for a hand held WCD utilizing such a prior art antenna, the antenna radiates electromagnetic radiation toward a human user of the WCD equipped with such an antenna as there is essentially no front-to-back ratio. For reference, the applicant notes that for multi-band versions of prior art types of antenna, the external half wave single or multi-band dipole antenna (i.e., where resonances are achieved through the use of inductor-capacitor (LC) traps), signal gain on the order of approximately a positive two decibels (+2 dBi) are common and expected.
In addition, due mainly to the inherent shape of such prior art antennas, when operating they are typically primarily sensitive to receiving (and sending) vertical polarization communication signals and may not adequately respond to communication signals that suffer from polarization rotation due to the effects of passive reflection of the communication signals between source and receiver equipment. Furthermore, such prior art antennas are inherently inadequate in sensitivity to horizontal polarization communication signals.
Another type of prior art antenna useful with portable wireless communication gear is an external quarter wave single or multi-band asymmetric wire dipole. This type of antenna operates much like the aforementioned external half-wavelength dipole antenna but requires an additional quarter wave conductor to produce additional resonances and, significantly, suffers the same drawbacks as the aforementioned half wave single band, or multi-band, dipole antenna.
Therefore, the inventor recognizes and addresses herein a need in the art of WCD antenna design for an antenna assembly which is compact and lightweight, that is less prone to breakage and has no moving parts (which may fail, become bent, and/or misaligned), and, which utilizes the available interior spaces and structure of a WCD to achieve a more compact final configuration.
There is also a need for a multi-frequency antenna assembly which is able to receive and transmit circularly polarized electromagnetic radiation at one or more preselected operational frequencies.
There is also a need in the art for a deformable antenna resonator which is equally responsive to a variety of different communication signals having a variety of polarization orientations.
There also exists a need in the art for an antenna assembly which is compact and lightweight and which can receive and transmit electromagnetic signals at one or more discrete frequencies and which antenna assembly can be tuned to one or more frequencies.
A turnstile antenna consists of two resonant dipoles at right angles to each other and crossing in the center. The two antennas are electrically isolated from each other. The main feedline, such as a 50 Ohm coax, is coupled to one dipole""s feedpoint connection. A 90 degree phasing line is provided between the one feedline connection to the other dipole feedline connection. The 90 degree phasing of the two dipoles is important toward obtaining an omnidirectional pattern.
The turnstile antenna is one of the many types that have been developed primarily for omnidirectional vhf communications. The basic turnstile consists of two horizontal half-wave dipole antennas mounted at right angles to each other in the same horizontal plane. When these tow antenna are excited with equal currents 90 degrees out of phase, the two antennas merge to produce a nearly circular radiation pattern.
Patch and quadrifilar helix antennas are used for applications such as GPS where circular polarization provides optimum link performance. Quadrifilar helix antennas are relatively large in size, and patch antennas, although much more compact, have the disadvantage of narrow bandwidth and are easily detuned due to their mode of operation.
The antenna of the present invention provides significant size advantages over known antenna structures, e.g, a smaller mounting footprint as compared to a patch antenna, and a height far less than a helix antenna, though somewhat greater than a dielectrically loaded patch. Electrical performance of the antenna of the present invention in a GPS application is similar to the helix and loaded patch antennas. An antenna according to the present invention is suitable for mass production. A dielectric base may be used, similar to a patch antenna; however, the material may be a low cost molded plastic for the present invention as opposed to a more expensive ceramic material.
A circularly polarized (CP) antenna is formed by a novel four arm resonator which may be placed relative to a conducting ground plane. The resonator has four conducting elements, each exhibiting quarter wave resonance in the band of interest. The elements are normally supported in a particular spatial relationship by a dielectric substrate or block, which is selected based on dielectric constant loss tangent, and thermal properties, as one skilled in the art would recognize. The elements are formed on the top and side surfaces of the dielectric block, and a microstrip transmission line with quarter wave delay portion is formed within the block, near the bottom. Two adjacent elements are fed with equal amplitude and 90 degree phase difference. The other two elements are parasitically excited from the opposite elements, with a 180 degree phase shift. A progressive phase shift of 90, 180, and 270 degrees between adjacent elements results in circular polarization. The primary feed location may be connected to a low impedance transmission line, becoming the input/output port of the antenna, and the other fed element is shunt fed through a quarter wavelength delay line. Shunt feed occurs on the sides of the block, near the ground plane, and the delay line is contained within the dielectric block. The feed system permits the use of a matching network if required, a feature not found in a patch antenna The resonator may be electrically connected to a conducting ground plane.
A microstrip or other type of transmission line on the ground plane may be used to feed the resonator. All electrical connections to the antenna may be surface mount type, which is facilitates automated installation. The antenna of the present invention may be manufactured at low cost and in high volume by a number of available methods. A two shot molded plastic with subsequent selective metallization is one, insert molded metal is another, and stamped metal parts attached to a dielectric block is a third.
An object of the present invention is to provide an antenna with elliptical and ideally circular polarization.
Another object of the present invention is to provide a circular polarization antenna having four elements, each element having two or more segments and exhibiting circular polarization derived at least in part from the geometry of the elements.
Another object of the present invention is to provide a circular polarization antenna having four elements, with two adjacent elements fed 90 electrical degrees apart, and their opposing elements parasitically excited with a 180 degree phase shift.
Another object of the present invention is to provide a circular polarization antenna of relatively small size, low cost, and suitable for high volume manufacture.
Another object of the present invention is to provide a circular polarization antenna suitable for surface mounting onto or within a wireless communication device such as a GPS receiver.
Another object of the present invention is to provide a circular polarization antenna constructed of conducting elements disposed on a dielectric base.
Yet another object of the present invention is to provide a circular polarization antenna which can easily accommodate an impedance matching network at its input/output port.